"The fact that they used MySQL does not make it any bettter."
Indeed. "Doesn't scale well" is what springs to mind, from experience.
MySQL is very good at what it's designed for (read heavy operations) but once you get past a few tens of millions of entries there are better DBs that have smaller memory/CPU footprints, which don't have these kinds of problems with replication.
People become highly resistant to change and attempt to keep banging the square peg into the round hole long after it's apparent it doesn't fit, rather than learn how to drive another DB. PostgreSQL and friends may seem big and scary upfront, but it's actually a helluva lot easier to run than trying to tune a large-scale MySQL installation - instead of lots of knobs, PgSQL tends to just get on with it.
(Personal experience: Same hardware, 500million entry DB, write heavy - PgSQL ended up about 5 times faster than MySQL and using 20% of the resources. Replication was a doddle too, so migrated to it. Next manager along didn't understand any of it, so ordered it ripped it out in favour of MySQL and then spent a year trying to make it work reliably.)